Mass, Spacetime, and Gravitons

Why does having mass mean you can warp spacetime? Are gravitons real or just an unproven theory?
- Muhammed (age 17)
Sir George Monoux College, London

Muhammed- More good questions!

1. I don't know why the curvature of spacetime depends on the mass density. So far as I know, no one knows yet. Perhaps all physical theories will always have a deepest level at which we can only say 'that's how things are'. The level is likely to go deeper, so maybe soon your question will be answerable on the basis of a deeper theory.

2. Gravitons have not yet been observed.* It is hard to see how they could not exist, since they are predicted on the bases of General Relativity and Quantum Mechanics. Both these theoretical structures have been found to have very wide domains of validity. They can’t be combined in a consistent way that would apply to all ranges of phenomena, but it seems very likely that there are no major tricks in combining them on the scale of, say, milliseconds, a range in which gravitational waves are being sought. Of course finding gravitational waves is not the same as finding gravitons, just as finding light didn’t right away mean finding photons.

You may find as you study science more that the line between things that are ’proven’ to exist and those that are ’unproven theories’ is not completely sharp.

Mike W.

*At one point the BICEP2 collaboration said they saw what looks like the leftover effects of the quantum zero-point spread of gravitational waves, left as tiny "B-mode" ripples in the polarization of the cosmic microwave background radiation. That turned out to be mistaken, but if more careful future observations turn up those waves, they'll have seen something that wouldn't be there unless gravitational waves were quantum mechanical, So in that sense, gravitons would be detectable. 

(published on 10/22/2007)